1. Field of the Invention
This invention relates to a metallic laminated sheet, and more particularly to a process for producing a heat resistant metallic laminated sheet composed of mutually isolated metallic sheets or foils and heat resistant, insulating films of heterocyclic polymer put between the metallic sheets or foils.
2. Description of the Prior Art
Previously metallic laminated sheets having metallic sheets or foils on both surfaces were usually produced by compressing and fusing an intervening film of thermoplastic polymer, such as, polyethylene, polypropylene, vinyl chloride and polyester, put between the metallic sheets or foils, or by sticking the metallic sheets or foils together with the binder. It is difficult to obtain a heat resistant laminated sheet using such polymers due to their low softening points. Various types of heat resistant heterocyclic polymers, for example, polyimide, polyamideimide, polybenzimidazol, polyhydantoin, polythiazol and polyimidazopyrrolone have been known to be suitable for producing the heat resistant film. The copolymers of said heterocyclic polymers are also suitable for use as heat resistant films. These films are known to have the excellent mechanical and electrical properties even in a temperature range below -40.degree. C and above 150.degree. C. These heat resistant heterocyclic polymers, however, are the condensation polymers, and are usually produced solution reactions, so that the heat resistant films must be produced by stretching and drying the polymer solutions. Since the resultant hardened film is not fused and insoluble in the solvent which is used in the synthesis, it is the usual practice to use precursors of the heterocyclic polymer or the unset coating or to use the other binder for laminating some metallic sheets or foils. Epoxy resin or the modified resin thereof, nitrile rubber binders, acetal binders and silicone resins are known to be useful as such binders, but these exhibit poor heat resistance as compared with that of the heterocyclic polymers. Accordingly, the heat resistance of the resultant laminated sheet can not exceed that of the binder itself.
On the other hand, fluorocarbon resins used as a binder for some laminated sheets have excellent heat resistance. Laminated sheets made with such fluorocarbon resin binders however, has the defect that printing and covering on the binder layer, if exposed, are difficult.
As a binder composed of the precursor of the heterocyclic polymer or the unset coating, there may be polyamide acid, low molecular polyimide prepolymer and polyamide imide. Polyamide acid, low molecular polyimide prepolymer or the like prepolymer generates a volatile condensed product in the bonding step. On the other hand, the polyamide imide unset binder, which sticks on the metallic sheets or foils due to the characteristic of lower distortion temperature compared with that of polyamideimide itself, brings out foaming or irregularity of the metallic surface and exhibits poor insulation, since the excess amounts of solvent for the unset material which are retained in the binder are difficult to expel in the production of the laminated sheet which is composed of metallic outside sheets and a binder layer put therebetween. Besides, even if an attractive laminated sheet is produced by compression at low temperature, the bonding effect of the binder will be broken by abrupt heating, due to excess amounts of residual volatile matter in the binder.